The present invention relates to a binder composition for bonding molds, shapes, forms, etc. The invention also relates to refractory molds, shapes and forms incorporating such a binder and to methods of producing and using the binder and refractory compositions.
Alkyl silicates and colloidal silica sols, among others, are two of the kinds of materials that have been used in binder compositions in preparing investment casting molds and other refractory molds, shapes and forms. Attempts have been made to combine these two materials to obtain the advantageous properties of each while minimizing the less desirable aspects. For example, ethyl silicate provides high refractoriness and can be quickly chemically hardened or air hardened, but it provides low bond strength and limited stability in refractory slurries. Colloidal silica sols, on the other hand, provides high bond strength and good slurry stability but cannot be chemically gelled to a single phase and is only slowly air hardened.
In U.S. Pat. No. 3,961,968 a hybrid system of alkyl silicate and colloidal silica is disclosed in which a binder composition including both alkyl silicate and colloidal silica is produced by using various solvents including alcohols, ethylene glycol monoethers and diglycol diethers. This binder composition provided a desirable hybrid binder product when gelled, and at low ethyl silicate levels of about 8% a stable binder composition can be prepared which can be gelled to a single phase. On the other hand, the hybrid binder does not have good long term stability when higher ethyl silicate concentrations are employed, i.e., the binder of a higher ethyl silicate concentration gels by itself after a short period of time without the addition of any gelling agent. Since higher ethyl silicate levels provide certain desirable characteristics for binder and refractory compositions, it would be desirable to provide a composition which is stable against self-gellation but also which includes high amounts of ethyl silicate and which gels on command to a single phase.
An improvement on the hybrid binder system of U.S. Pat. No. 3,961,968 is disclosed in U.S. Pat. No. 4,289,803 in which phosphoric acid is included in the hybrid alkyl silicate/colloidal silica binder composition to produce a phosphosilicate composition, which is disclosed as enhancing mold strength and refractoriness of the binder composition. The binder compositions of U.S. Pat. No. 4,289,803 are stable at low ethyl silicate levels but not at high ethyl silicate levels.